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Brain’s map of space falls flat when it comes to altitude

8 August 2011

Animals' brains are only roughly aware of how high-up they are in space, meaning that in
terms of altitude the brain’s ‘map’ of space is surprisingly flat, according to
new research.

In a study
published online in Nature Neuroscience, scientists studied cells in or
near a part of the brain called the hippocampus, which forms the brain’s map of
space, to see whether they were activated when rats climbed upwards.

The study,
supported by the Wellcome Trust, looked at two types of cells known to be
involved in the brain’s representation of space: grid cells, which measure
distance, and place cells, which indicate location. Scientists found that only
place cells were sensitive to the animal moving upwards in altitude, and even
then only weakly so.

Professor
Kate Jeffery, lead author from UCL Psychology and Language Sciences, said: “The
implication is that our internal sense of space is actually rather flat – we
are very sensitive to where we are in horizontal space but only vaguely aware
of how high we are.

“This
finding is surprising and it has implications for situations in which people
have to move freely in all three dimensions – divers, pilots and astronauts for
example. It also raises the question – if our map of space is flat, then how do
we navigate through complex environments so effectively?”

How the
hippocampus makes its map of space is fairly well understood for flat
environments, but the world is of course not flat – it has a richly varied
topography, and a useful map therefore needs to work in all three dimensions.
However, adding a third dimension to the two horizontal ones makes things very
much more complicated for a map, and it is not clear how – or even if – the
brain can encode this.

To begin to
answer this question scientists looked at neurons known as grid cells, which
become active periodically and at very regular distances as animals walk around,
forming a grid-like structure of activity hot-spots. Previous work has found that grid cells are
largely concerned with marking out distances.

In the
study, rats walked not just on flat ground but also on pegs on a climbing wall,
or else on a spiral staircase, so that the rats moved not only horizontally but
also vertically. Interestingly, the grid cells still kept track of horizontal
distance but did not measure out vertical distances. It seems as if grid cells do not “know” how
high they are.

In the
second part of the study scientists looked at another type of neurons known as
place cells. Place cells, found in the hippocampus itself, produce single
activity hotspots in the environment and seem to function to encode specific
places. These neurons were only weakly sensitive to height too – but they did
show some responsiveness, suggesting
they received information about height from some other, possibly non-specific,
source.

Professor Jeffery
said: “It looks like the brain’s knowledge of height in space is not as
detailed as its information about horizontal distance, which is very
specific. It’s perhaps akin to knowing
that you are “very high” versus “a little bit high” rather than knowing exact
height.”

Image: Firing pattern from a grid cell recorded on a flat surface. Credit: Kate Jeffery/UCL